1. Field of the Invention
The present invention relates to a semiconductor device formed in a nitride semiconductor layer made of a group III-V nitride semiconductor and a manufacturing method of the same.
2. Description of the Related Art
Recently, in compound semiconductors, an HEMT (hereinafter, referred to as GaN-FET) using a heterojunction between an AlGaN layer and a GaN layer and having a group III-V nitride semiconductor layer including a GaN layer made as an electron transit layer has been widely developed. GaN is a material having a wide band gap, a high breakdown field strength, and a high saturation electron velocity, so it is considered to be promising as a material for high-voltage operations and high-power devices.
In such a compound semiconductor device for high-voltage operations, it is required to reduce gate-leakage current. As a gate electrode of a GaN-FET, a schottky electrode made of Ni, Pt and the like is now used. In this case, however, there is a problem that gate-leakage current is generated when the gate voltage is increased in a forward direction. As a method for solving this problem, there is a MIS-type GaN-FET having a gate insulation film of insulation film made of SiO2, Al2O3 and the like.
With reference to FIG. 6, a conventional MIS-type GaN-FET (a first conventional example) will be described.
By a known MOVPE method, an electron transit layer 102 having a film thickness of about 3 μm and an electron supply layer 103 made of an intentionally undoped AlGaN (for example, Al0.25Ga0.75N) of intentionally undoped GaN, having a film thickness of 20 nm, are sequentially deposited on a sapphire substrate 101.
Next, after a source electrode 104 and a drain electrode 105 are formed by using, for example, Ti/Al, a gate insulation film 106 made of SiO2, Si3N4 and the like is formed. Then, a gate electrode 110 is formed on the gate insulation film 106 by, for example, a lift-off method. With this process, a MIS-type GaN-FET according to the first conventional example is completed.
According to the first conventional example, since SiO2 and Si3N4, which are used as a material of the gate insulation film 106, do not have a high dielectric constant, problems such as shifting of a threshold value to a reverse direction or reduction in mutual conductance can occur. To solve these problems, it is effective to apply oxide having relatively high dielectric constant, such as Ta, Hf, Zr, to a gate insulation film.
With reference to FIG. 7, a MIS-type GaN-FET (second conventional example) having a gate insulation film made of a high dielectric constant material will be described.
An electron transit layer 102, which is made of an intentionally undoped GaN and having a film thickness of about 3 μm, and an electron supply layer 103, which is made of an intentionally undoped AlGaN (for example, Al0.25Ga0.75N) and having a film thickness of 20 nm, are sequentially deposited on a sapphire substrate 101 by using a known MOVPE method.
After a source electrode 104 and a drain electrode 105 are formed as layers, a gate insulation film 111 made of Ta2O5 and the like is formed as an insulating material having a high dielectric constant. Then, a gate electrode 110 is formed on the gate insulation film 111 by, for example, a lift-off method. With this process, a MIS-type GaN-FET according to the second conventional example is completed.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-324813
[Patent Document 2] Japanese Patent Application Laid-Open No. 2006-108602
As described above, according to the MIS-type GaN-FET disclosed in the second conventional example, since the gate insulation film is made of an oxide of a high dielectric constant material such as Ta, Hf and Zr, shifting of a threshold value and reduction in mutual conductance can be prevented. On the other hand, however, using a gate insulation film made of an oxide causes following problems. When an insulation film, here, a gate insulation film, is deposited on a nitride semiconductor layer, an interface state is generated between interfaces of the nitride semiconductor layer and the gate insulation film. The interface state causes an electron trap so that the amplification characteristic of the semiconductor is deteriorated.
Further, in addition to the above problem, there is a problem that is specific to a group III-V nitride semiconductor device such as a GaN-FET. That is, nitrogen is often separated from a nitride semiconductor layer during a manufacturing process. Particularly, when such a nitrogen separation occurs in an area under a gate electrode, a leak current can be increased. To solve this problem, nitrogen can be compensated to the nitride semiconductor layer where nitrogen separation occurs during the manufacturing process; however, this solution requires additional or more complicated processes.